1. Field of the Disclosure
The present invention relates to apparatus for storing energy, and particularly but not exclusively to apparatus for receiving and returning energy in the form of electricity (hereinafter referred to as “electricity storage” apparatus).
2. Description of the Related Art
A number of systems have been proposed for electricity storage that store the heat of compression of air and absorb the work of expansion of air.
A commonly proposed example of this is called Adiabatic CAES where a salt cavern is typically used as a compressed air store. When electricity is to be stored a motor drives a compressor to compress air into the cavern. The compression process raises the temperature of the air and to allow efficient energy recovery it is necessary to store this ‘heat of compression’ in some form of thermal store.
The cavern will normally be kept at a minimum pressure, such as 40 bar, and this is increased to a higher limit, for example 60 bar, during charging. These pressures are likely to generate a peak temperature, using air, in the region of 650 degrees C. This is normally either transferred to an unpressured thermal store by a heat exchanger or stored directly in a thermal storage matrix contained within a pressurised vessel. To recover the electricity the process is reversed and the compressed gas is reheated by the thermal store prior to expansion. The work of expansion is used to drive a generator to generate electricity.
If a heat exchanger is used rather than a thermal storage matrix in a pressurised vessel, the aim is to store the heat with only a small difference between the compressed air temperature and the storage material temperature, such that when the process is reversed the air is heated to near its original temperature.
This sort of heat exchange is extremely difficult to achieve because there are no heat transfer liquids that operate in the range 0-650 degrees C. This means that either multiple liquids must be used or the heat exchange is via a gas, which means a gas to gas heat exchanger.
Multiple heat transfer liquids are difficult to manage, require multiple storage vessels and are generally expensive, but they can operate efficiently and avoid the cost of heavily pressurised vessels.
With gas to gas heat exchangers the temperature range requires the use of quality steels and the gas flows require very large heat exchangers to avoid pressure drop. The result of this is that these heat exchangers are normally both very expensive and not very efficient, with a large temperature difference, such as 50 degrees C., after each heat transfer process.
The most efficient solution is to use a thermal storage matrix, such as a particulate structure, contained within an insulated pressure vessel and to transfer the heat to and from the gas in a manner that is similar to a very large regenerator. This has the best heat transfer, but the storage mass must all be contained within the pressure vessel, which is very expensive.